1. Field of the Invention
The invention relates to a white light-emitting device, and more particularly to a light-emitting device including one white phosphor that can absorb light emitted from a semiconductor light-emitting chip to generate a white light.
2. Description of the Related Art
White light is a light mixed from a plurality of lights of different colors. Visible white light is one generated by mixing least two lights of different wavelengths. For example, when the eyes are simulated by red, blue and green lights at the same time, or by blue and yellow lights at the same time, they recognize the incident light as a white light. A light-emitting diode (LED) light source is made with this theory in mind. Four processes are commonly used to generate a white light for a convention LED device. The first process uses InGaAlP, GaP and GaN to make LED devices that respectively control currents passing through LED devices to generate the red, green and blue lights. Since these three LED devices are placed in one lamp, a lens of the lamp mixes the lights emitted from the LED devices to generate white light. The second process uses GaN and GaP to make two LED devices for controlling an electrical current passing the LED devices to emit blue and yellow-green lights, respectively. The blue and yellow-green lights are mixed to generate a white light. These two methods provide 20 lm/W. However, if one of the LED devices responsible for providing a specific color of light is non-functional, then white light will typically not be obtained. Furthermore, since positive biases applied on these LED devices are different, several control circuits to control the biases are required, causing an increase in production costs. The third process was developed by Nichia Chemical Company, Japan, in 1996, and uses InGaN blue diode and yellow yttrium aluminum garnet powders to provide white light. The process currently provides 15 lm/W, which is less than that provided by the above two processes; however, only one LED device is needed. This process has been successfully commercialized due to the mature technology of preparing the phosphors powder. The second process and the third processes implement the complementary color principle to generate white light. The continuity of the spectrum wavelength distribution is not as good as that of sunlight. Therefore, white light obtained by mixing lights appears non-uniform color in the visible light range (400 nm–700 nm), resulting in low color saturation. Although the human eye can neglect the phenomenon and just see white light, high-precision optical detecting equipment such as a camera or a picture shooting device has a color rendering property. That is, errors may occur when colors of an object return to their original conditions. Therefore, white light generated by such a process is only suitable for simple illumination applications. A fourth process was developed by Sumitomo Electric Industries, Ltd, Japan in 1999. In the fourth process, a CdZnSe film is formed on a ZeSe single-crystal substrate to emit a blue light. The blue light also radiates on the substrate to emit a yellow light. The blue light and the yellow light form complementary colors and generate white light. In this process, one LED device is used and the operational voltage is only 2.7V, rather smaller than the 3.5V needed by the LED device formed on GaN. No phosphor is needed for obtaining white light. However, its main disadvantage is that it provides only 8 lm/W of illumination, and the service life thereof is only 8000 hours, which limits the applications thereof.
In the currently used LED devices, three or more phosphors are stimulated to emit specific lights that are then mixed to generate white light. They are potentially replacements for fluorescent lamps or bulbs in the future. Stimulating the phosphors to emit light requires specific exciting lights that can be exactly absorbed by the phosphors at the same time. Therefore, there cannot be significant difference between the absorption coefficients of the phosphors for the exciting lights. Quantum efficiencies in optical energy conversion for the phosphors are preferably as similar as possible. Such a complicated process is not the best candidate for obtaining white light.